Although a great variety of latices have been developed which are useful as finish coats, binders, adhesives, back-coatings, transfer films and interlayers for a wide variety of textile applications, there lacks a latex which, alone or in cooperation with other materials used in textiles:
(1) improves the stiffness of the textile material under elevated temperatures and high relative humidity; PA1 (2) is acid resistant; and PA1 (3) is coagulable under specified temperature conditions.
These properties are essential for the polymers used in battery separators.
Battery separators are thin, porous webs which are impregnated with a latex binder and passed through drying ovens to remove the water. In a battery cell, closely spaced metal electrode plates are connected in series and are immersed in a highly acidic, electrolytic solution. The battery separators are placed between the metal electrode plates to prevent the metal electrode plates from contacting each other and to prevent metal salts or other conductive materials from forming bridges between the metal electrode plates. Both of these problems would ultimately short circuit the cell. The battery separator must remain sufficiently porous to allow the free flow of electrolyte solution between the metal electrode plates for effective ion exchange.
When the battery separators are inserted between closely spaced metal electrode plates, flexibility of the separator can cause problems by preventing the separator from being smoothly inserted between the metal electrode plates causing a disruption in the assembly operation. At elevated temperatures and at high relative humidity, the problem worsens.
Battery separators are manufactured commercially from a wide variety of fibers such as, for example, cellulose, glass, polyolefin, polyester and the like, fillers such as, for example, diatomaceous earth, different clays, silica, quartz, hydrocarbon polymer powders and the like, bound together with an organic binder supplied as a latex or aqueous dispersion. Battery separators made with conventional latex binders have a degree of stiffness which decreases as temperature and relative humidity increase. The decrease in stiffness causes manufacturing problems during assembly of the batteries.
A number of patents have addressed the need for improved battery separators. For example, U.S. Pat. No. 4,529,677 discloses a novel, improved, battery separator material particularly adaptable for use in maintenance-free batteries. The battery separator material includes a diatomaceous earth filler, an acrylate copolymer binder which includes a silane coupling agent attached to the polymeric backbone, and a combination of fibers comprising polyolefin, polyester and glass fibers. The acrylate copolymer binders contain about 80 weight %, or less preferably from about 80 weight % to about 30 weight % of a C.sub.1 to C.sub.8 alkyl acrylate monomer. The copolymer has a glass transition temperature of from about 30.degree. C. to about 60.degree. C. Furthermore, U.S. Pat. No. 4,363,856 discloses organic binders for battery separators. The binders are conventional, commercially available, film forming polymers based on monomers, such as methacrylic acid, acrylic acid, ethyl acrylate, methyl acrylate and the like which result in hydrophilic, flexible binders.
One approach to a stiffer battery separator has been to design one-stage, latex binders based on monomers which give a stiffer, more hydrophobic polymer. When this has been done by incorporating monomers such as, for example, styrene, alkyl substituted styrenes or isobornyl methacrylate in place of methyl methacrylate in a conventional latex used as a binder for battery separator plates, the complex factors controlling the coagulation temperature of the formulated latex binder on the nonwoven mat were disrupted and coagulation did not occur at the desired temperature from about 30.degree. C. to about 60.degree. C., the accepted coagulation temperature range in the industry, with temperatures from about 40.degree. C. to about 45.degree. C. preferred.
When the composition of the latex binder is adjusted to obtain a stiffer battery separator, the new composition may not coagulate within the desired temperature range. Battery separators are formed from fiber and fillers into a nonwoven mat which is then impregnated with a latex binder. The whole assembly is then dried at elevated temperatures to crosslink the latex binder and evaporate the water from the latex binder, thereby forming the battery separator. During the drying operation the latex binder tends to migrate to the surface of the battery separator as the water evaporates resulting in a nonuniform distribution of the latex binder. Therefore, to avoid this problem, latex binders are carefully formulated so that they are stable during impregnation of the nonwoven mat, but they coagulate uniformly throughout the nonwoven mat during oven drying at a low and narrow temperature range before any significant amount of water has evaporated. For latex binders currently in commercial use in battery separators, formulations have been developed wherein the binders coagulate at temperatures from about 30.degree. C. to about 60.degree. C., preferably from about 40.degree. C. to about 45.degree. C.
Multi-stage polymers have been employed for use on textiles to improve low temperature properties, such as flexibility. For example, U.S. Pat. No. 4,107,120 teaches latex compositions of a core/shell morphology and their use with textile materials to improve the low temperature properties thereof. U.S. Pat. No. 4,277,384 further improves upon that invention by providing latex compositions of a core/shell morphology and their use with textile materials which not only enhance the low temperature properties but also improve the flexibility and resistance to seam tearing of the textile materials. U.S. Pat. Nos. 4,181,769 and 4,351,875 disclose, respectively, the articles of manufacture of said core/shell compositions. None of these approaches, however, teach the use of a multi-stage latex binder composition wherein one stage enhances the stiffness of the textile fibers and another stage controls the coagulation temperature of the latex binder. This invention not only satisfies the need for stiffer textile materials under elevated temperatures and high relative humidity, especially in an acid environment, but also fulfills the need for a latex binder whose coagulation temperature may be controlled.
It is therefore an object of this invention to provide an improved textile fiber which contains an acid-resistant, coagulable, multi-stage latex binder.
It is a further object of this invention to provide a method for improving the stiffness of textile materials.
Other objects and advantages will become apparent from the following description and claims.